Of course you need to sell your product but as a builder you can do it yourself. It's not your specialty so you likely will be worse at it than a dedicated person and will have less time for actual building.
The key is that builders can exist without salesmen. But salesmen without builders have nothing to sell.
I am against some specializations getting paid per unit of work regardless of the market value of their product while other specializations get paid a cut of the market value.
I am against positions of power which allow people who don't produce anything to decide how much other people who do actually produce something get paid.
Etc.
I am fundamentally against inequality.
One who can both build and sell is indeed a rare specimen (unicorn?). I know many builders and less than 10% of them can sell.